The projection of a light beam by a lighting device of a motor vehicle, such as the front headlamps or rear lights, conventionally allows the road to be illuminated with an overall illumination and thus the visibility in the dark to be improved, for example at night. This allows the vehicle to be driven safely. These lighting devices may comprise one or more lighting modules.
The regulations impose that the light beams projected by the lighting modules comprise a cut-off so as not to dazzle the other users. This cut-off, which takes the form of a “cut-off line”, creates a transition region between a lit region and a dark region. The shape of the cut-off line is generally regulated, for example the regulation ECE n°48 in Europe or the regulation FMVSS 108 in the USA.
When a light beam with a cut-off line is projected by a conventional lighting module, a coloured effect is observed in the transition region. This observation results from the phenomenon of chromatic dispersion of light and of the dispersive nature of the optical components integrated into the lighting module. In practice, the main consequence of this phenomenon is that the lamps projecting a beam of white light appear blue for the other users; this phenomenon is called chromatism. Furthermore, this chromatism can be a nuisance for drivers.
One known solution is for example to use an achromatic system composed of concentric circular diffractive elements. These diffractive elements allow the chromatism of the cut-off line to be corrected in several directions. Nevertheless, this solution requires a significant quantity of diffractive elements, in other words a large part of the lens must thus be covered by these diffractive elements. Thus, a lighting module comprising a lens with such concentric circular diffractive elements exhibits a limited diffraction efficiency which results in a loss and scattering of the light from the light source. Moreover, the concentric diffractive elements make diffraction spots appear due to the other diffraction orders in the dark region: these spots are a nuisance, notably for the other users. Furthermore, the larger the number of diffractive elements, the more complex the system to be fabricated which increases its cost of production.
In this context, there exists a need to improve the projection of a light beam with a cut-off line and to facilitate the fabrication of the diffractive element.